Electrostatic spray coating system



Aug. 15, 1961 .1. w. JUVlNALL 2,996,042

ELECTROSTATIC SPRAY COATING SYSTEM Filed Feb. 11, 1955 INVENTOR.

JAMES W. JUV/A/ALL BY Z'EM 4 Attorneys nular sources.

2,996,042 ELECTROSTATIC SPRAY COATING SYSTEM James W. Juvinall, Indianapolis, Ind., assignor to Ransburg Electra-Coating Corp., Indianapolis, Ind., a corporation of Indiana Filed Feb. 11, 1955, Ser. No. 487,498 3 Claims. (Cl. 118-624) This invention relates to the coating of articles of manufacture with a spray of electrically charged liquid coating material particles and more particularly to methods and apparatus for controlling the distribution of spray particles moved through a quiescent atmosphere and deposited on the surface of an article under the infiuence of an electrostatic field. The term quiescen is here used in the sense that the atmosphere or air trite States Patent through which the charged spray particles move as they paths of the spray particles and their distribution over the surface of the article being coated.

United States Patent No. 2,736,671, issued February 28, 1956 discloses a system of electrostatic spray coating in which the spray particles are atomized from the circular edge of a rotating bell-shaped atomizing head and are projected in a direction generally axial of such edge by an electrostatic field maintained between the atomizer and an article being coated. The pattern in which the spray from such an annular-edged device is deposited on a stationary flat surface of relatively large area maintained substantially normal to the axis of the atomizer (which pattern will hereinafter be referred to as a static pattern) is an annulus having a center portion substantially devoid of spray particles. If the flat surface referred to is moved rectilinearly through the spray in its own plane the spray will form on the surface a band of coating material characterized generally by the fact that it increases in thickness for a distance from its longitudinal center toward its side edges. The cross-sectional shape of a band so produced will hereinafter be referred to as a dynamic pattern. In some instances, the dynamic pat tern produced by an unmodified annular spray may be such that the thickness at and adjacent the center of the band is half, or even less than half, that at points nearer the sides of the band.

The prior patent of I I. W. Juvinall, Patent No. 2,839,425, dated June 17, 1958, and of R. C. Juvinall et al., Patent No. 2,877,137, dated March 10, 1959, discuss at greater length the characteristics of static and dynamic patterns produced by unmodified annular sprays and disclose expedients for improving uniformity in the thickness of coatings produced by sprays emanating from an- However, the devices there disclosed possess certain disadvantages, one of which is the fact that they are not well adapted to conveyorized operations in which articles to be coated are moved through a coating zone in spaced relation and the atomizer is triggered, or so controlled as to be operative only when an article is in position to receive the discharged spray.

It is an object of this invention to provide improved control over the distribution of electrically charged particles projected in a general axial direction from an annular-edged atomizer. Another object is to promote uniformity in the thickness of the coating produced on extended surfaces moved through a spray having a normal annular shape, and to do so in a manner which overcomes disadvantages present in prior systems aimed at improving uniformity in the coating produced by normally annular sprays. A further object is to provide an electrostatic coating method and apparatus in which a normally annular spray is modified to better adapt it for III Patented Aug. 15, 1961 triggered control. Still another object is to produce'an electrostatic spray-coating apparatus, and an atomizer adapted to use in such apparatus, for coating the opposed surface portions of adjacent articles moved in spaced relation through a coating zone.

In practicing my invention in its preferred and most generally used form, I provide an annular-edged atomizing device, such as one of the usual type disclosed in the aforesaid E. M. Ransburg patent, No. 2,73 6,671, capable of projecting a normally annular spray of electrically charged particles in a general axial direction, and I associate such atomizing device with a conveyor adapted to move through the axially projected spray the article or articles to be coated. To modify the distribution of the electrostatically deposited spray particles, whereby to attain the objects above noted, I employ an electrode which provides spray-repelling zones near the annular edge of the atomizer and effectively confined to regions located adjacent the ends of an edge-diameter extending transversely to the path of article movement. Such spray-repelling zones improve uniformity in the thickness of coatings produced on extended surfaces and provide a spray which is well adapted to triggered control. In addition, the spray-repelling zones tend to divide the spray along a plane extending axially of the atomizer and transverse to the path of article movement. Where it is desired to coat opposed surfaces of adjacent articles moved through the coating zone in spaced relation, the atomizer may be arranged for movement into spaces between adjacent articles, and in such an arrangement the spray-spilling tendency of the spray-repelling zones promotes a uniform division of the spray between the adjacent articles.

The principles and details of my invention will be fully understood from the following description of an embodiment thereof, with reference to the accompanying drawings wherein:

FIG. 1 is a isometric view illustrating apparatus for practicing my invention in the coating of automobile bodies;

FIG. 2 is a fragmental side'elevation, in partial section, illustrating one of the atomizers shown in FIG. 1;

FIG. 3 is a plan view illustrating in somewhat idealized form the static pattern produced by one specific embodiment of my invention; and

FIG. 4 is a view similar to FIG. 3 showing the static pattern produced by another embodiment of my invention.

In the apparatus illustrated in FIG. 1, automobile bodies 10 are passed in succession and in spaced relation through a coating zone by a conveyor diagrammatically indicated at 11. At each side of the conveyor there is a pair of atomizers 13 positioned to discharge a paint spray horizontally against the lower portion of the side of the automobile body and a second pair of atomizers 14 arranged to discharge paint sprays horizontally against the upper portions of the automobile-body sides; and above the path followed by the automobile bodies there is another atomizer 15 arranged to project a paint spray downwardly to coat the tops of the bodies. In the particular apparatus illustrated, it is contemplated that the atomizer 15 will be mounted for vertical movement under control of a fluid pressure cylinder 16 so that it may enter the space between adjacent automobile bodies and discharge coating material onto the opposed front and rear surfaces thereof.

The several atomizers 13, 14, and 15 may be of different sizes, but otherwise their construction may be similar. For purposes of describing that construction, I have illustrated in FIG. 2 one of the atomizers 14. As there shown, the atomizer includes a rotatable atomizing hell or head 17 having a peripheral atomizing edge 17a and mounted for rotation on one end of a hollow shaft of a motor (not shown) mounted within a housing 18. When the motor is energized, bell 17 will rotate about a line 17x which is the axis of the atomizer and the bell. Liquid coating material is fed from a reservoir at a controlled rate through a conduit 20 to a stationary feed tube 21 running through the hollow motor shaft and projecting into the atomizing bell 17. Attached to the end of feed tube 21 is a hollow cylindrical tube 22 of metal with a cut-away section 22a to permit free flow of liquid from the-end of feed tube 21 to the interior'surface of atomizing bell 17. This particular feeding arrangement, which forms no part of my invention, is more fully described and claimed in the US. Patent No. 2,728,606 issued December 27, 1955.

Attached to the end of tube 22 is a two-ended electrode 24 conveniently made of metal rod about threesixteenths of an inch in diameter and preferably having hemispherically tipped terminals 24a and 24b. Electrode terminals 24a and 2412 are positioned adjacent diametrically opposite portions of atomizing edge 17a. The atomizer 14, or at least the head 17 and electrode terminals 24a and 24b, is maintained at a high electrical po tential relative to autobody 11, as by connecting the atomizer to the high voltage terminal of a high-voltage source 26 and grounding the autobody through its con- ,veyor.

In the operating of the atomizer, power supplied to the motor within housing 18 will cause the rotation of atomizing bell 17 about its axis 17x. It is apparent that while bell 17 rotates electrode 24, which is attached to feed tube 21 by means of tube 22, will remain stationary. Liquid coating material fed at a controlled rate from a reservoir through conduit 20 to stationary feed tube 21 will be discharged from the lip of tube 21 onto the inner surface of the rotating bell. The liquid thus fed to the surface of the bell will be formed by centrifugal force into a thin film flowing outwardly over the inner surface to the atomizing edge 17a, where it will be atomized under the influence of the electrostatic field maintained between edge 17:: and the surface-f body to form a spray of electrically charged particles which will be moved to and deposited on the body 10 by the aforesaid field. As initially formed, the spray of charged particles is annular in cross-section, and would tend to retain that annular cross-sectional form if it were not for the electrode 24.

The electrode 24 modifies the spray by effecting a redistribution of the spray particles both parallel to and perpendicular to the axial plane in which the electrodes lie. The nature and extent of such redistribution will depend upon several factors including principally the radial and axial location of the electrode tips 24a and 24b. In FIG. 3 I have illustrated a slightly idealized static pattern produced by one specific atomizer having the following characteristics:

The static pattern of FIG. 3 had a width (extent parallel to the median line 32) of about forty inches.

It will be noted from FIG. 3 that the static pattern, instead of being a uniform annulus, comprises two generally similar, arcuate lobes 30a and 30b symmetrical abouta common or longitudinal center-line ,31 :and sym 4 metrically disposed on opposite sides of a transverse median line 32 with their concave edges presented toward such median line. In FIG. 3, it has been attempted to illustrate only the outlines of the static pattern, which are rather ill defined; and no attempt has been made to illustrate the distribution of the spray particles within thecutlines. Enough of the circumferential momentum of the coatingmaterial supplied to the edge of the atomizer head 17 persists to cause the median line 32 to be displaced angularly from the axial plane occupied by the electrode 24. In the specific example illustrated in FIG. 3, such displacement amounted to about 10 and was, of course, in the direction of head-rotation.

If an extended flat sheet having a transverse extent greater than that of the static pattern is moved along the longitudinal center-line 31 in its own plane, the dynamic pattern produced on such sheet, instead of possessing the relatively thin center portion characteristic of dynamic patterns produced by unmodified annular sprays, will be of a generally trapezoidal shape, having a center portion of substantial transverse extent and relatively uniform thickness. If the sheet has a width somewhat less than the transverse extent of the static pattern, as is the case in the sheet 35 illustrated in FIG. 3, the marginal portions of the spray near the ends of the lobes 30a and 3011, which spray-portions would form the extreme tips of the trapezoidal dynamic pattern, will be drawn inwardly by the electrostatic field existing over the face of the sheet, and the sheet will receive a substantially uniform coating from one edge to the other. If the sheet has a width greater than can conveniently be coated to uniform thickness with a single atomizer, a plurality of atomizers may be employed, offset from each other laterally of the path of sheet movement to the extent necessary to cause the tips of the trapezoidal dynamic patterns of adjacent atomizers to overlap slightly. The use of overlapping dynamic patterns of trapezoidal shape to build a coating of uniform thickness is more fully illustrated and described in the Juvinall et al. patent above referred to.

The improved uniformity in the thickness of the dynamic pattern produced by my invention is due to a lateral redistribution of the particles in the normally annular spray. Some of the particles directed toward surface portions which would be coated to abnormal thickness by an unmodified spray are usually deflected inwardly or toward the center of the dynamic pattern, and some are deflected outwardly. The outward deflection increases the overall width of the dynamic pattern to a slight extent and also decreases the slope of its inclined sides. The latter feature is of advantage when wide surfaces are coated with a plurality of sprays, as it renders less critical the extent to which the dynamic patterns of adjacent sprays overlap in the production of a reasonably uniform coating.

The longitudinal redistribution of particles, or redistribution in the direction parallel to the longitudinal center-line 31, has an advantage when the atomizer is triggered. In triggering, it is customary to initiate sprayprojection after the article being coated has partially entered the normal static pattern of the spray and to terminate spray projection befor'e the article has moved completely out of such static pattern. If the spray were initiated before the article entered the normal static pattern and maintained until the article cleared such pattern, considerable coating material might be lost; and on the other hand, if the spray is initiated too late or terminated too early, the leading and trailing portions of the article will not receive an adequate coating. In pra tice, an atomizer employed to coat a sheet 35 moving in the direction indicated by the arrow in FIG. 3 might be triggered to terminate the spray when the sheet reaches the position illustrated. It will be apparent from FIG. 3 that the sheet will be deprived of the paint represented by the end of the static pattern, or that portion thereof which lies to the left of the trailing edge of the sheet. A spray may be, and prior electrostatic sprays from annular sources have been, so shaped that disuniformity in the distribution of coating material along the trailing edge of the sheet would result or be increased by failure of the sheet to receive the pain-t represented by an end portion of the static pattern. This is true of sprays which produce static patterns that are annular or that are of the hollow polygonal shape disclosed in the aforesaid R. C. luvinall et al. patent, for in such sprays the end portions, or at least one end portion of the static pattern, are concentrated within a fraction of the width of the entire pattern, and the failure of the sheet to receive them would result in a localized reduction in coating thickness. With my invention, the longitudinal rearrangement of the spray particles is such that particles near the sides of the static pattern-i.e., near the ends of the lobes 30a and 30bare displaced away from the median line 32 to an extent greater than are particles near the longitudinal center-line 31. As a result the paint of which the trailing edge of the sheet 35 is deprived when the atomizer is triggered is paint which would have been reasonably well distributed transversely of the sheet, and the effect of triggering in creating disuniformity of coating thickness along the trailing edge of the sheet is materially reduced if not practically eliminated.

The same advantage obtained by my invention in triggering at a trailing edge is of course likewise obtained in triggering at a leading edge; since the static pattern is essentially symmetrical about the transverse median line 32.

It is not essential in the practice of my invention that the electrode 24 be so constructed and arranged as to split the static pat-tern into two substantially discrete lobes, such as are shown in FIG. 3. In many instances, such for example as that represented in FIG. 4, the static pattern approximates an annulus notched internally at diarnetrically opposite points and having an outer periphery which is substantially continuous. The static pattern of FIG. 4, like that of FIG. 3, had awidth of about forty inches and Was produced under the same conditions except that the electrode tips 24a and 24b were spaced one-quarter inch ahead of and one-half inch inwardly from the atomizing edge 17a. The dynamic pattern produced by the spray of FIG. 4 was markedly more uniform than that produced by an unmodified annular spray but somewhat less uniform than that produced by the spray of FIG. 3.

Even when, as in FIG. 4, the electrode 24 produces a relatively small effect on the outline of the static pattern its tendency to split the spray can nevertheless produce a substantial effect on the distribution of particles within that outline. Where the static pattern is not completely or substantially split, a comparison of maximum or average thickness along the median line with maximum or average thickness along diameters angularly remote from the median line is at least a rough measure of the effectiveness of the electrode 24 in providing desirable alteration of the dynamic pattern. Some benefits of my invention might be obtained with an electrode which leaves maximum thickness along the median line of the static pattern greater than one-fourth that in other regions of the pattern; but in the average case, maximum improvement in coating uniformity would not be expected unless the electrode was capable of reducing median-line thickness to one-fourth, or less than one-fourth, that found elsewhere in the pattern.

The tendency of the electrode 24 to divide the spray in the manner just described is of particular advantage when, as in the case of the atomizer 15 shown in FIG. 1, an atomizer is advanced into the region between two spaced articles to coat opposed surfaces of such articles; as in such a situation, the transversely arranged electrode promotes an equal division of the spray between such opposed surfaces.

The precise location of the electrode tips 24a and 24b is subject to some variation. So far as concerns the beneficial redistributing effect of the electrode 24 its tips may lie on the radius of the spray-source or well inwardly or outwardly therefrom. However, projection of they tips radially beyond the atomizing edge 17a tends to cause a deterioration in the quality of electrostatically produced atomization, and excessive radial projection may in addition result in the coating of the tips, which is undesirable. Deterioration of atomization may be expected if the electrode tips lie outside a hypothetical conical survface coaxial with the edge 17a, passing therethrough, and having an included apical angle of 30, and electrode coating may be expected if the tips lie outside a similar conical surface having an included apical angle of Electrodes 24 having a diametric extent as small as twothirds the diameter of the edge 17a have been found effective in promoting a beneficial redistribution of spray particles. Axially, the electrode tips may be located close to the plane of the edge 17a or well beyond such plane in the direction of spray-projection. Excessive axial spacing between the electrode tips and the discharge edge 17a is undesirable, as it reduces the electrical potential which can be safely applied at the edge 17a and thus weakens the field at that point. With an atomizing head twelve inches in diameter, I have obtained excellent results with an electrode having tips 24a and b located two inches ahead of the edge 17a and on the radius of such edge; but within the practical limits imposed by the necessity for maintaining atomization of desired quality and the desirability of avoiding coating of the electrodes 24, there is a wide range within which the electrode tips can be located while still effecting an improved distribution of the atomized particles in the spray.

The tendency of rotation of the head 17 to cause an angular displacement of the median line of th static pattern relative to the plane of the electrode 24 has been noted above. Where uniform distribution of coating material on continuous, extended surfaces is desired, the plane of the electrode 24 is preferably displaced from a plane perpendicular to the direction of article travel by an angular interval such that the median line 32 of the static pattern at the article surface will lie in the latter plane. In some instances, however, it is desirable that the static pattern of the spray be divided otherwise than at a plane perpendicular to the path of article travel. For example, the presence of the window openings in the upper portion of the automobile bodies illustrated in FIG. 1 may make it desirable to orient the electrode 24 associated with one or each of the heads 14 so that the splitting of the static pattern will be along an inclined plane to promote a division of the spray between the upper and lower edges of the window openings.

In the particular atomizer illustrated in FIG. 2, the head 17 and electrode 24 are directly connected electrically and will therefore have the same potential. This feature is not necessary; as the head and electrode might have different potentials so long as that of the electrode is effectively spray-repelling.

In addition to being better adapted to a triggered operation than are prior arrangements designed to improve uniformity in the coating produced by annular sprays, my invention possesses other important advantages, including simplicity of construction and an improved capability of functioning satisfactorily in spite of variations in operating conditions, such as the spacing between atomizers and work, the type of coating material handled, and the rate at which such material is supplied to the atomizer.

It will be understood that the specific embodiments of my invention shown and described above are set forth merely by way of example and that wide changes in construction and arrangement may be made without departing from the spirit and scope of the invention as defined in the claims.

I claim as my invention:

1. Apparatus for electrostatically spray coating an article, comprising an atomizer having an annular atomizing edge lying concentrically about an axis of said atomizer, means for moving the article to be coated along a predetermined path axially spaced from said atomizing edge, means including a source of high voltage for creating an electrostatic field between the moving article and coating material at said annular atomizing edge, means for feeding liquid coating material at a controlled rate to said annular edge for atomization therefrom as a hollow spray of liquid particles and projection toward the article to be coated, a single electrode mounted on the atomizer having but two end portions located within the spray and on diametrically opposite sides of the spray axis, said end portions lying substantially in a single plane extending axially of the atomizer and transverse to said path of article movement, and means for maintaining said electrode end portions at a particle repelling potential to reduce the concentration of spray particles in diametrically opposite regions of the spray adjacent such plane and increase the concentration of spray particles in two similar bands located on opposite sides of and extending generally parallel to such plane.

2. The apparatus of claim 1 including means for moving a plurality of articles in succession and in spaced relationship along said path and means for sequentially moving said atomizer into the space between adjacent articles and withdrawing said atomizer from said space as the next article approaches the atomizer, whereby the spray concentrated in one of said bands is deposited on the retreating face of one article and the spray concentrated in the other band is deposited on the approaching face of the succeeding article.

3. Apparatus for electrostatically spray coating an article, comprising an atomizer having an annular atomizing edge lying concentrically about an axis of said atomizer, means for moving the article to be coated along a predetermined path axially spaced from said atomizing edge, means including a source of high voltage for creating an electrostatic field between the moving article and coating material at said annular atomizing edge, means for feeding liquid coating material at a controlled rate to said annular edge for atomization therefrom as a hollow spray of liquid particles and projection toward the article to be coated, means providing only two electrode end portions located within the spray and on diametrically opposite sides of the spray axis, said end portions lying substantially in a single plane extending axially of the atomizer and transverse to said path of article movement, and means for maintaining said electrode end portions at a particle repelling potential to reduce the concentration of spray particles in diametrically opposite regions of the spray adjacent such plane and increase the concentration of spray particles in two similar bands located on opposite sides of and extending generally parallel to such plane.

References Cited in the file of this patent UNITED STATES PATENTS 2,083,634 Brackett June 15, 1937 2,097,233 Meston Oct. 26, 1937 2,203,851 Tucci June 11, 1940 2,221,338 Wintermute Nov. 12, 1940 2,302,289 Bramston-Cook Nov. 17, 1942 2,434,125 Schweitzer Jan. 6, 1948 2,447,374 Smyser Aug. 17, 1948 2,509,448 Ransburg May 30, 1950 2,658,009 Ransburg Nov. 3, 1953 2,658,472 Ransburg Nov. 10, 1953 2,710,773 Sedacsik June 14, 1955 2,777,419 Raasche Jan. 15, 1957 2,780,565 Juvinall Feb. 5, 1957 2,877,137 Juvinall et al. Mar. 10, 1959 FOREIGN PATENTS 591,474 Great Britain Aug. 19, 1947 679,751 Great Britain Sept. 24, 1952 OTHER REFERENCES Ransburg: No. 2 Process, Ransburg Electrocoating Corp., Indianapolis, Ind., 1953, 15 pages. 

